The 5K is 5 kilometers, about 3.11 miles. It is short enough to race hard and long enough that pacing still matters: most runners who fade in a 5K went out too fast in the first kilometer.
Enter your goal time to see the pace per mile or per kilometer you need, then use the splits table to check yourself at each marker. For example, a 25:00 5K needs 5:00 per kilometer, or about 8:03 per mile.
If you don't have a goal yet, enter the pace you run comfortably in training and the calculator shows the finish time that pace would give you.